The present invention relates to a variable ratio bicycle chain drive. More specifically, the present invention relates to a variable ratio bicycle chain drive having a sprocket assembly with a variable radius.
The vast majority of multiple speed bicycles being produced today are of the derailliur/gear cluster design. That design includes an arrangement whereby a chain is selectively engaged with different gears. To change the gear ratio, the derailliur "derails" the chain from one gear and the chain is engaged to another gear. This basic design has been unchanged in recent history.
A variation on the basic derailliur/gear cluster design has used one or more fixed radius elliptical gears or sprockets such that the effective gear ratio varies with the bicycle rider's leg position.
A product was recently introduced which uses a front radial drive gear which is capable of expanding and contracting in diameter in order to change the gearing ratio. This new design eliminates the need for bulky gear clusters in the rear of the bicycle. Disadvantageously, changing to a lower gear ratio requires back pedaling, thereby interrupting the drive power.
The following patents show various adjustable diameter mechanisms:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 758,474 Reed 4/26/04 1,126,195 Granz 1/26/15 4,030,373 Leonard 6/21/77 4,065,032 Lydiksen 12/27/77 4,068,539 Nye 1/17/78 4,129,044 Erickson et al. 12/12/78 4,493,678 Husted 1/15/85 -4,521,207 Husted 6/04/85 ______________________________________
The Reed patent shows a speed changing mechanism having radially adjustable rim sections using disks with spiral grooves.
The Granz patent discloses a pulley having adjustable rim sections mounted upon worm gears and having bevelled wheels coaxially attached to the worm gears.
The Leonard patent discloses a variable diameter transmission arrangement wherein a number of movable sheave elements are adjustable in corresponding tracks by way of friction wheels turning pinions. The pinions engage gears to rotate threaded rods upon which the sheave members are mounted.
The Lydiksen patent shows a container filling machine having a pinion gear which drives a threaded shaft upon which a cam is mounted. The pinion cooperates with one of two rows of teeth in order to adjust the cam position upon the threaded shaft.
The Nye patent discloses a variable transmission including an adjustable diameter pulley cooperating with a friction belt. One of the embodiments uses radially extending worm gears upon which adjustable traveler blocks are mounted. At interior ends of the worm gears are bevel gears which are coupled together by a central gear for rotation in unison.
The Erickson patent has a variable gear ratio chain drive system wherein chain engagement elements are mounted upon shafts which extend through channels within two plates. The two plates are moved along the length of the shafts so as to vary the position of the chain engagement elements, thereby changing the radius of the sprocket.
The two Husted patents disclose a variable radius sprocket arrangement wherein a number of chain engagement elements may be secured at various locations along corresponding spiral slots, the chain engagement elements moving in radial keyways. The spiral slots are in a wheel normally coupled for rotation with the wheel having the radial keyways, but the wheel with the spiral slots may move relative to the other wheel by use of a brake system, thereby allowing one to change the gear ratio depending upon the pedaling of the bicycle. These patents generally correspond with the commercial product referred to above wherein back pedaling is required to change the gear ratio in a particular direction.
Although previous variable ratio bicycle chain drives using variable radius sprockets have provided a wider variety of gearing ratios than the more commonly used derailliur/gear cluster arrangements, such prior arrangements have often been subject to one or more of several disadvantages. They are often complex in construction, thereby increasing difficulty in the cost of manufacturing and assembly. Some prior variable radius sprockets for bicycle transmissions require extra wide construction, thereby sticking out substantially from the bicycle frame. Some designs are sufficiently unusual in operation and/or structure as to require bicycle frame modification. A further disadvantage of some designs is that the rider must change his or her pedalling in order to change gears. Such an interruption of drive power makes such prior systems unsuitable for racing bicycle designs. Another disadvantage of many prior art designs is that they add excess weight to the bicycle, again limiting the usefulness of such transmissions. An additional disadvantage of some prior art designs is that they do not permit the chain to align properly in all gearing ratios. This skewed, or non-linear tracking produces the grinding and chain "chatter" commonly associated with multi-speed bicycles.